Sensing devices having orthogonal arrays are well known in the art for radars, sonars and microphones. A pioneering design, the Mills Cross, was built in the 1950s in Australia and utilized in a telescope comprising 250 dipole elements on two 1500 foot long arms, one running North-South and the other running East-West. Multiplying the voltages of the two arms produced a pencil beam with substantial sidelobes, and by adjusting the phasing of the elements in each arm, the telescope beam could be steered across the sky. Other systems utilizing the Mills Cross design include a Doppler radar in Norway, described by Singer et al. in “A New Narrow Beam Doppler Radar at 3 MHz for Studies of the High-Latitude Middle Atmosphere,” and “A New Narrow Beam MF Radar at 3 MHz for Studies of the High-Latitude Middle Atmosphere: System Description and First Results.” The Singer radar embodies the classic Mills Cross structure of transmit and receive elements in both planes, therefore the system does not produce a cross-product of the transmit and receive apertures. The present invention, in contrast, discloses transmit apertures in one plane and receive apertures in an orthogonal plane, which produce a cross-product of the two orthogonal apertures.
A number of patents disclose orthogonal arrays for transmitting and receiving sonar waves. U.S. Pat. No. 4,121,190 to Edgerton et al. describes a method of sonar location having a narrow beam angle in a first plane and a wide beam angle in an orthogonal plane, to provide wide-angle echo-detection in the orthogonal plane with narrow-angle discrimination in the first plane. The Edgerton design simultaneously transmits and receives in both planes, therefore the product of those two beams does not produce the same image as processing the beams independently, as is disclosed by the present invention. U.S. Pat. No. 5,323,362 to Mitchell et al. discloses an ultrasound sonographic system having an orthogonal Mill's Cross scanner array in which high resolution scanning is performed by a synthetic orthogonal line array. A receiving transducer element (hydrophone) and a transmitting transducer element (projector) are moved from spot to spot along their respective orthogonal array lines. U.S. Pat. No. 6,084,827 to Johnson et al. discloses an apparatus and method for three dimensional tracking of underwater objects, having one multibeam sonar head in a first plane, a second multibeam sonar head in a second plane that intersects the first plane, for receiving sound waves, and a sound wave transmitter.
Orthogonal antennas are also known in the art. For example, U.S. Pat. No. 3,521,286 to Kuecken discloses at least three mutually orthogonally radiating elements which are substantially decoupled and may be independently tuned over wide operating frequency ranges. The intent of this invention is to use the orthogonally polarized elements to increase transmit and receive isolation, so that the transmit and receive elements can operate at the same frequency. The two horizontal elements and one vertical element are co-located (overlapping) and cross each other at a neutral point that keeps the elements from interfering with each other, unlike the present invention, which does not disclose co-located elements. As such, the Kuecken invention does not provide a cross-product to the orthogonal transmit and receive element, and thus does not disclose the functionality of the present invention.
Radars having separate transmit and receive apertures are known in the art. For example, frequency-modulation continuous-wave (FM/CW) radars typically comprise separate transmit and receive apertures in order to achieve high isolation between the transmitted signal and the receive signal reflected off the target. Typically, the transmit and receive apertures are the same size and point in the same direction in azimuth and elevation. In order to increase the resolution and range of the radar system, both apertures may be made larger. In the present invention, however, the transmit and receive apertures are orthogonal, and resolution and range may be increased by increasing aperture length in one dimension, and then taking the cross-product of the independent transmit and receive patterns.
Radar systems with linear antennas are well known in the art, dating back to the first wartime air defense system, the Chain Home radar system developed in Britain in the 1930s. The advent of parabolic reflectors enabled radars to transmit and receive a narrower, more focused beam and therefore use energy more efficiently. Further advances in antenna technology introduced phased array antennas into radar systems, wherein electronic steering eliminated moving parts that thus enabled faster scanning and made the devices much more reliable.
The present invention is directed to an innovative solution that achieves high resolution at lower cost, higher reliability, and/or smaller footprint than known designs: an antenna system wherein the apertures are substantially orthogonal to each other and separately perform the transmit and receive functions. The cross-product of the transmit and receive apertures of the present invention thus provides a narrow spot beam and a higher resolution image than that produced by conventional apertures that both transmit and receive.
As disclosed herein, the present invention may comprise at least two orthogonal antennas, wherein at least one is a transmit aperture and at least one is a receive aperture, and wherein the apertures may be of various shapes, including horn; pill box; planar; dielectric lens; dielectric rod; Cassegrain; or parabolic, elliptical or circular dish. By virtue of their orthogonal orientation, the cross-product of the two apertures is a higher resolution spot beam. The resulting antenna is beneficial because it may be smaller and lighter than conventional designs, and thus take up less surface area when installed. This then allows room for other sensors or antennas.
The antenna system of the present invention may alternatively comprise at least two orthogonal antennas, wherein each aperture rotates on a one-axis gimbal, and at least one is a transmit aperture and at least one is a receive aperture. The receive and transmit apertures scan in orthogonal planes.
The present invention may also comprise at least two orthogonal linear phased array antennas, wherein at least one is a transmit aperture and at least one is a receive aperture, and wherein the transmit and receive apertures scan in orthogonal planes. For example, the antenna system of the present invention may comprise a first 1D array that scans in a vertical (used herein interchangeably with “elevation”) orientation and a second 1D array scans in a horizontal (used herein interchangeably with “azimuth”) orientation. Various known methods of scanning may be employed by the present invention to scan the linear transmit aperture and the linear receive aperture, including mechanical scanning, electronic beam switching, electronically scanned phased array and digital beamforming.
It is well known that radars employing phased arrays benefit from a variety of system performance enhancements. Such benefits include beam agility; ability to form multiple beams; and packaging and form factors (conformal or low profile). The main cost drivers for phased arrays typically are the module cost and the cost of integration of the modules into the phased arrays. By using an innovative orthogonal linear array, the present invention offers comparable performance to conventional 2D filled arrays at a cost savings of from 5 times to 50 times or even more in larger arrays. In many radars, performance may be limited by the beamwidth (clutter) of the system and the necessity to generate and track multiple targets. At the same aperture size, the present invention provides comparable clutter reduction to that of a 2D filled array, by increasing the length of the 1D arrays by a factor of less than 1.5. A high resolution is achieved in the region overlapped by the two orthogonal fan beams generated by the two orthogonal apertures. In this innovative solution, two orthogonal beams with wide aspect ratios are combined to achieve a narrow spot beam product. By tapering the sidelobes and increasing the length of the arrays (by approximately 35%), as compared to the linear dimension of a 2D filled array, very similar clutter and 2-way sidelobe structure may be achieved.
As disclosed herein, each 1D array of the present invention may comprises a plurality of antenna elements disposed on any suitable array face, which may be a substrate, ground plane, boom, vehicle, rooftop, soil, or floating in water. The antenna elements, also termed herein phased array elements, may either transmit or receive or may comprise both transmit and receive modules, which then may be switched between transmit and receive functions. As disclosed herein, the antenna elements may be conventional elements that comprise a radiator, an amplifier, a switch, a phase shifter, and control electronics for various phase shift control functions. The antenna elements preferably are formed onto an array mounting fixture that has certain conductive and dielectric properties that define the bandwidth, frequency of operation, directivity, and polarization responses of the elements, depending on the desired application of the radar system. As disclosed herein, the array mounting fixture may be formed from metal, dielectric, string, an inflatable surface, cloth or other suitable material, or may be placed directly on the ground. Signals of each antenna element are combined through the combining network that comprises amplifiers and phase shifters.
As disclosed herein, the present invention combining network may be either analog or digital. A typical analog combining network may comprise coaxial cable in a space-fed combining network, wherein the signal is transmitted through air or other dielectric medium to the receive or transmit receptacle on the array element. As contemplated herein, forms of analog signal combining may include microstrip, strip line, twin lead, and wave guide. The present invention may also be directed to a digital beamforming combining network, wherein A/D converters are employed to send a digital signal to a computer or microprocessor and mathematically produce the various beam states of the array as part of the digital algorithm.
The present invention thus discloses a radar system wherein the transmit signal is reflected from a target or other object and is received by the orthogonal array, such that the 2-way transfer function results in the cross-product of two antenna patterns (one vertical and one horizontal). For the linear array embodiment, this cross-product is substantially the same as the product resulting from a fully populated 2D scan array. The output of the combining network is transmitted into a radar processing receiver, and ultimately may be displayed in various ways, such as a radar display, an audio alarm, or a warning light or other optical output. As embodied herein, the present invention may operate with a variety of radar waveforms, including frequency modulated continuous wave (FMCW), CW and pulse Doppler.
The following well-know radar formula describes the cross-product of the present invention:
      P    receive    =                    P        transmit            ⁢              G        transmit            ⁢              G        receive            ⁢      σ      ⁢                          ⁢              λ        2                                      (                      4            ⁢                                                  ⁢            π                    )                3            ⁢              R        4                            Where Ptransmit is the power of the transmit signal; Gtransmit is the gain of the transmit antenna; Greceive is the gain of the receive aperture; σ is the radar cross-section (reflected signal from the target); λ is wavelength; and R is the radius to target.        
Applications for the present invention include radar altimeters and obstacle avoidance; brown-out radars; missile guidance; missile defense radars (for example, when disposed on a tall ˜300 meter structure); missile homing radars (for example, when formed as a circular conformal row of elements and another elongated linear array); ordnance/missile fuzing; weather radars (for example, when disposed on a long tower); wind profilers (for example, when disposed on two long orthogonal sticks); use with phase shifters; multiple beams (Butler matrix or Rotman lens); digital multibeam; space applications (for example, when flown on two long sticks in V or X shape); and search radar (for example, when disposed on two long sticks); fire control radars; airport traffic radar; vehicle collision avoidance; and light detection and ranging (LIDAR).
A preferred embodiment of the present invention may be employed as an affordable, high-resolution lightweight brownout landing aid for helicopters, overcoming limitation of prior art radars. As is well known, the acoustic, vibration and shock levels imposed on a helicopter from environmental and operational conditions are much more severe than those imposed on other air platforms. Using known technologies, a helicopter pilot's landing and takeoff aids have been dominated by optical frequency sensors at both the visible and IR frequencies. Known systems have degraded and/or limited range in adverse weather and brownout sand and dust storm conditions, however, that have limited the flight safety in desert and high precipitation environments. These limitations can also leave a helicopter open to other risks and vulnerabilities, including trap wires strung between buildings and trees when common ingress and egress paths of a helicopter are known. Urban/suburban landing and takeoffs can also become dangerous if nearby mobile land vehicles are in close proximity to a makeshift helicopter landing site. For example, where these mobile land vehicles have limited visibility to approaching aircraft in a tactical brownout environment, the vehicles may not be able to move out of the way of the landing helicopter, and it may be difficult for the incoming helicopter to detect the mobile vehicles. Other ground-based human activities in urban operations can also interfere with a helicopter's safe landing. Microwave and millimeter wave (MMW) imaging systems offer the advantages of a lower frequency range that can see farther in range, and such systems are less affected by severe atmospheric changes. A radar system also offers full day/night capability without performance degradation, and in particular, a MMW radar system offers the resolution required to determine safe landing and takeoff conditions, as well as a package size that can be incorporated within the weight and size constraints of military and commercial helicopter platforms. For cost and technology maturity reasons, mechanically scanned MMW antenna systems are often considered for helicopter landing applications, but such systems must be designed to operate with high reliability and extremely fast scanning rates in order to meet the landing and full 360° coverage requirements in azimuth over the full range of dynamic conditions of the helicopter. The logistics, maintenance, and support of the mechanically scanned antenna systems often become the most important cost driver and the limiting factor of the system. An electronically scanned phased array is the ideal choice for the above requirements for rapid scanning, lower profile, and reliability. The limitation then becomes the cost of the MMW phased array.
Any MMW radar system must also compete for the same real estate on the undercarriage and sides of the aircraft as the other RF systems, including UHF Line of Sight (LOS), data links, altimeters, navigation, IFF, and other communications systems antennas. The end result produces a considerable real estate competition/shortage and/or platform antenna(s) integration issue. These issues may include interference and blockage from multiple single function RF apertures that often will degrade the radars stand-alone and modeled performance. Thus, in addition to weight and cost considerations, a major challenge is the need to find the optimum way to integrate the radar antenna's functionality onto the helicopter platform while allowing for multiple simultaneous RF functions to exist, all without degradation to either the radar's stand-alone performance or that of the other RF systems.
As described herein with reference to FIGS. 10, 11, 12 and 13, the MMW radar system 5 of the present invention provides an innovative RF multi-function capability that enables the integration of new sensor technology onto the helicopter while maintaining existing system effectiveness. As embodied herein, the present invention provides an antenna system architecture that can incorporate multiple functions (like those described above) into a single antenna system that will result in lower cost, weight, and reduced number of apertures on an aircraft. The solution must be small, lightweight, low physical volume, visually concealed, and have a low radar cross section (RCS), while simultaneously performing each antenna function without degradation to the primary antenna(s) function. This is accomplished by the innovative technology of the present invention, based on the volumetric reuse of the area that would have been occupied by a 2D filled aperture. The present invention provides fast scanning as well as fine resolution, achieved from the product of two transmit and receive beams.
In order to achieve desirable cost, weight and performance objectives of a MMW Radar antenna system, the present invention contemplates two orthogonal electronically scanned/multiple beam antennas with an approximately 5° beamwidths in one plane and fan beam in the orthogonal dimension. This allows for rapid scanning in both azimuth and elevation, and the ability to determine the radar return at multiple ranges on 5°×5° pixel by pixel basis. This is achieved by generating the cross product of the elevation and azimuth scan positions of the two orthogonal arrays. As embodied herein, radar system 3 uses a low power MMW frequency. It is also possible with this design to generate simultaneous receive beams to reduce update times, thus minimizing transmit power requirements for the radar system. Analysis of the waveform shows that a single channel radar with a total effective isotropic radiated power of 100 mW at MMW waves is sufficient to detect objects with 3 m2 Radar Cross Section (RCS) at an operating altitude of 150 meters. The angular resolution preferably is set at 5°. Narrower beamwidth and higher angular resolution can be achieved with linear (as opposed to square) dependency on the number of elements and the length of the arrays, as described further below. As such, Applicant believes that the innovative design of the present invention overcomes the cost barrier of a 2D scanned array in this application for helicopters.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and, together with the detailed description, serve to explain the principles of the present invention.